This invention relates to a tool radius compensation system and, more particularly, to an apparatus and method useful in compensating for the radius of a tool in a three-dimensional machining operation.
A numerical control device usually comes equipped with a tool radius compensation function. Compensation of tool radius is accomplished by moving the center of a tool along a path that is offset to the left or right of a programmed path by a distance equivalent to the tool radius, thereby compensating for a cutting error attributed to the radius. For example, with reference to FIG. 1, assume that a programmed path is defined by two straight lines L1, L2 which intersect at an angle .alpha. of between 90.degree. and 180.degree.. The tool radius is compensated for in this case by prereading a move command located in the present block of data b.sub.1 and a move command located in the next block of data b.sub.2, deriving straight lines L1', L2' obtained by offsetting the straight lines L1, L2 in the respective blocks b.sub.1, b.sub.2 by the tool radius (offset quantity) r, and computing the coordinates of the point S1 at which L1' and L2' intersect. Then, when the tool is transported by a pulse distribution operation from an end point So in the previous block to the end point S1 in the present block, the center of the tool will traverse a path offset by the radius r from the correctly commanded programmed path, causing the workiece to be machined in the manner commanded.
Providing a numerical control device with a tool radius compensation function of the kind described simplifies programming immensely because the programmer need not take tool radius into consideration when creating a numerical control tape. Moreover, when tool radius changes owing to wear or because one tool has been substituted for another, the operator need only set the value of the new radius using a radius setting dial provided on an NC panel, or enter the new radius value by means of a manual data input unit (referred to as an MDI). Either of these operations stores the new value in memory so that machining can be performed correctly based thereon.
The case described above relates to a tool radius compensation performed in two dimensions. For a three-dimensional radius compensation, data indicating the amount of compensation along each of three axes must be included in each block of data, unlike the foregoing case for two dimensions. This makes it impossible to create an NC tape that includes three-dimensional offset data without making use of an automatic programming apparatus.
With regard to the last-mentioned point, there is available a so-called 21/2-dimensional machining method which combines two-dimensional contour machining and a so-called pick-feed operation to achieve three-dimensional machining of a workpiece. In such 21/2-dimensional machining, moreover, there are cases where, during the machining of a contour defined by sectioning the workpiece by a predetermined plane, there is almost no change in the inclination of the machined surface along said contour. In cases such as these, where it is unnecessary to specify compensation direction in each and every block of data, programming in three-dimensions is quite feasible without relying upon an automatic programming apparatus and without the laborious task of designating compensation direction block for block.